Everyone wants faster Internet access, and for many, that means getting DSL. In the previous issue ("Know Your DSL," November 7, 2000), we explained how DSL works and looked at the varieties available. In this issue, we'll take a more practical approach and look at what you need to get DSL service working in your home. This is a process I went through recently, and my experience may prove instructive.

Four factors typically determine whether you'll be able to get DSL: whether your central office (CO) has been upgraded for DSL; whether your local loop has a device called a digital loop carrier; the distance from your home to your CO; and the quality of your local loop.

You begin by supplying a potential service provider with your telephone number. This is used to locate the CO that serves your home to see whether it has been upgraded for DSL. The CO houses equipment and wiring that connect you to your ILEC's (Incumbent Local Exchange Carrier) or CLEC's (Competitive Local Exchange Carrier) backbone DSL network. That network connects you to your ISP, which then connects you to the Internet. If the CO equipment and connections haven't been installed yet, you'll have to wait before you can get DSL.

If your local loop has a digital loop carrier (DLC), you also might have to wait. A DLC is a device installed in a cabinet near the curb (see the first diagram) that multiplexes local telephone line traffic for delivery to the CO via a high-speed connection, often fiber-optic cable or a T1 line. DLCs must be upgraded in order to carry DSL. Most providers are only just beginning to test and deploy DSL solutions on DLC loops.

Assuming your CO has been upgraded and you're not sidelined by a DLC, your provider will use test equipment to measure the electrical characteristics of the loop. The results are used to estimate the length of the wire from your home to the CO. This measurement is important, because DSL is distance-limited. So this distance calculation will determine whether you can get DSL and, if you can, what services your provider can offer you. For example, Verizon's (formerly Bell Atlantic) full-rate ADSL offering, called Info-speed DSL, required that my home be within 15,000 feet of the CO. The company estimated that my home was 14,200 feet from the CO, which put me rather close to the limit.

Finally, your line is tested to determine whether it is of sufficient quality to support DSL service. Attempting to turn on service only to find out that a line can't carry DSL is a costly mistake. Some lines, for example, have devices called load coils,which extend a line for ordinary telephone service but block DSL. Bridge taps, which are branches off the main line, also affect DSL performance. Even the type or thickness of the copper wire used can have an effect.

My loop included a device called a digital added main line, or DAML, that had to be removed before the line could be qualified. DAMLs are sometimes used to provide a second analog telephone line where an additional line is needed, but an actual second wire pair is unavailable.

With the DAML removed, my line was loop qualified for two download service rates: 640 Kbps and 1.6 Mbps. ADSL is usually delivered rate adaptive, which means the service automatically adjusts the throughput rate to line conditions, so providers typically quote maximum values for downstream and upstream rates. For example, the Bell Atlantic Personal Infospeed plan offered 640 Kbps downstream and 90 Kbps upstream for $49.95 per month at the time this was written, which included an Internet ISP account. Your actual performance will vary, however, up to the maximum.

Though I qualified for the Personal Info-speed (640 Kbps downstream) and Profes- sional Infospeed (1.6 Mbps downstream) service offerings, I did not qualify for the Power Infospeed service, which offers up to 7.1 Mbps downstream and up to 680 Kbps upstream. Higher-speed ADSL requires a shorter local loop distance. The Power Infospeed offering requires a local loop length of 12,000 wire feet or less, and mine was 14,200 feet.

DSL service requires equipment on the customer's premises and in the CO. In your home, you need a DSL modem that typically attaches to your PC via a standard Ethernet adapter and that attaches to your DSL telephone line via ordinary telephone cable. If your PC already has an Ethernet card for a local area network, you'll need another one for the DSL modem. Some DSL modems support PCI or USB connections, in which case you won't need the Ethernet card.

On the CO side, the equipment includes a digital subscriber line access multiplexor (DSLAM), whose purpose is to aggregate traffic from all of the incoming DSL lines and feed it to the DSL backbone network, usually via high-speed ATM lines. The DSLAM houses a rack of line cards, where each card serves some number of customer DSL lines. The line cards contain DSL modem circuitry on-board, so your local loop connection is terminated on your side with a DSL modem and on the CO side with a DSL modem.

Choosing a DSL Modem

At the time, Bell Atlantic offered two ADSL modem choices: an external Westell Wirespeed and an internal Efficient SpeedStream 3060. I ordered the Westell, because it's easier to move from one PC to another and because its diagnostic LEDs make troubleshooting easier.

Your choice of equipment will depend on your provider. Even though ADSL is standards-based, and in theory, most equipment should work, ILECs and CLECs typically partner with equipment providers whose products they have tested and decided to support.

Splitting Voice From Data

The Westell modem arrived as part of a package that included additional hardware and an excellent picture guide with instructions. The package also contained a number of microfilters, which you place between your analog devices and the wall jacks. The microfilters block the high- frequency data signal used by DSL, so you can use your telephone, fax, answering machine, and even an analog modem on the DSL line. The first diagram shows the placement of the microfilters.

This ability to use a single line for both DSL and ordinary analog telephones is a key feature of ADSL. Implementation differs, however, depending on the provider and your installation. Micro-filters have the advantage of not requiring any wiring changes. Sometimes, an external splitter is used. This device is typically installed next to your network interface device (NID, which is where the external telephone wire enters your home) and splits voice and data upon entry into the home. In this case, your service provider will usually install new inside wiring to your DSL modem. This is shown in the second diagram.

Regardless of the technique used on the customer's premises, a splitter is also used at the CO. Data is sent to the DSLAM and routed to the Internet, and voice traffic is sent to a telephone switch and the public switched telephone network (PSTN).

When the technician arrived, I had already installed the modem, so he verified that my line was DSL-enabled, and we installed the microfilters. He then used a device called a service verification set to connect to the CO and test the sync speed. He verified that downstream bandwidth of 608 Kbps was available. That translates to roughly 76 KBps (kilobytes per second).

That completed the hardware installation. A day later, someone arrived to install the software, but I had done that already. This involved installing Bell Atlantic's version of Netscape and the PPPOE (Point-to-Point Over Ethernet) protocol software required to obtain a session. The company uses PPPOE for session establishment and authentication and then assigns you a temporary Internet Protocol address when you log on. Your vendor may use different browser software, temporary or permanent IP addresses, or PPPOE.

I then followed the instructions for establishing my account with the Bell Atlantic.net ISP. I prefer Microsoft Internet Explorer for my browser, so I entered the appropriate settings in Internet Explorer and removed Netscape from my computer. Everything worked smoothly, and at this point I was, for the most part, finished.

Profile, Please

My service wasn't as fast as it should have been. I tested my throughput using several Internet measurement sites (www.computingcentral .com/topics/bandwidth/speedtest50.asp, http://speedtest.mybc.com, and www.2wire.com/dlp/dlp_bandwidth.html) and was averaging only 240 Kbps. The company said I should be achieving approximately 80 percent of the downstream bandwidth, or 486 Kbps, which meant I was getting only half the expected throughput. So when the software installer arrived, I had him call around and look for explanations.

We discovered that my profile had not been properly set up yet. According to the provider, there are over a dozen profiles set up at various places that hold settings and parameters defining your DSL service. For example, a profile tells the network what ISP you should be routed to and what service type you have. Generally, the absence of a profile is an indirect measure of whether your service order has been fully assigned and processed. We quickly got the problem fixed, and I sent the software technician on his way. Unfortunately, fixing the profile had zero effect on performance.

Fine-Tuning TCP/IP

After a little research and testing on my own, I discovered the solution to the throughput problems. The TCP/IP stack in Windows 98 Second Edition employs a number of configuration settings that are stored in the Registry. These settings are geared toward dial-up modem users and must be retuned for high-speed DSL. For example, DefaultRcvWindow defines the size of the receive window employed by TCP. (Note that if you don't find this key, you can add it.) This value defaults to 8,192 and tells the sending system how much data to send before expecting an acknowledgement. On systems that use high-bandwidth connections, this value must be set higher to avoid introducing an artificial bottleneck. Setting mine to 32,688 instantly doubled my throughput. A number of other settings needed changing, as well. For more information on this topic, see the Registry Tweaks page on www.speed-guide.net.